Sodium hydroxide and chlorine that are important industrial materials are manufactured mainly by chlor-alkali electrolysis methods.
The present electrolysis process has progressed through a mercury process using a mercury cathode, a diaphragm process using an asbestos diaphragm and a soft iron cathode to an ion exchange membrane process using an ion exchange membrane as diaphragm and an activated cathode involving small overvoltage. Through such transition periods, the power consumption for manufacturing one ton of caustic soda has decreased to 2000 kWh.
An activated cathode for hydrogen evolution is obtained, for instance, by the following methods and materials: the method to obtain an active electrode by composite plating in Ni plating bath in which active carbon powder is dispersed; the method by Ni alloy plating from the plating bath containing a second element, like S or Sn; the method of activation by NiO plasma spray or Pt—Ru displacement plating for Ni surface; the method by porous Ni applying Raney nickel; the method by preparing a Ni—Mo alloy film by arc-ion plating process; and the method by impregnating hydrogen storage alloy to provide resistance to reverse electric current. (Refer to Non-Patent Literature 1.)
Recently, an electrolytic cell that can increase current density for the purpose of increasing production ability and decreasing an investment cost is now under development in an ion exchange membrane process. Development of a low resistance membrane enables large current to apply.
However, the cathode so far used has large surface unevenness and low mechanical strength of the catalyst layer, without established records about its life and performance as a cathode for the ion exchange membrane process. Then, the following requirements for improvements are raised. In order to realize a new process, it is essential to develop an activated cathode having high performance and sufficient stability even under the afore-mentioned electrolytic conditions. It is further required for the activated cathode to have a low overvoltage, not to impair a membrane by contacting and to be low contamination with, for example, metal ions from a cathode.
In the chlor-alkali process most generally conducted, an activated cathode for hydrogen evolution is arranged so as to contact with the surface of a cation exchange membrane, or to have a gap of 3 mm or lower from the surface of the ion exchange membrane. On the catalytic layer of the anode and the cathode, chloride ions react with water to form chlorine gas and sodium hydroxide. Anodic reaction and cathodic reaction are as follows, respectively.2Cl−=Cl2+2e (1.36V)2H2O+2e=2OH−+H2 (−0.83V)
Theoretical decomposition voltage is 2.19V.
However, where the conventional cathode is operated at a large current density, there are some large problems, for example, as follows.    (1) Part of a substrate (nickel, iron or carbon component) dissolves and peels due to deterioration of an electrode, and such a component migrates into a catholyte, a membrane or an anode chamber, resulting in deterioration of product quality and deterioration of electrolysis performance.    (2) Overvoltage increases with increasing a current density, resulting in decreasing energy efficiency.    (3) Distribution of gas bubbles in a cell increases with increasing a current density, resulting in causing distribution in concentration of sodium hydroxide formed. As a result, solution resistance loss of a catholyte increases.    (4) Where operating conditions are severe, the amount of impurities (sulfur, iron or the like) effused from a cell constituting material increases, resulting in contamination of an electrode.
It is expected that a constitution that an activated cathode for hydrogen evolution is arranged so as to closely contact with an ion exchange membrane (zero gap) can decrease voltage and such a constitution is desirable. However, this constitution has the possibility that a membrane is mechanically broken by a cathode having a rough surface. Thus, there has been the problem to use the conventional cathode at a high current density and under zero gap condition.
In order to solve the above-mentioned problems, encountered when the activated cathode by the conventional methods are used, the inventors of the present invention have developed an activated cathode for hydrogen evolution, as shown below, as the one by the thermal decomposition process.    (1) An activated cathode with the mixed catalyst of cerium and precious metal coated on the surface of nickel substrate (Patent Literature 1)    (2) An activated cathode with a precious metal coating layer and a cerium coating layer laminated on the surface of nickel substrate (Patent Literature 2)    (3) An activated cathode with a base coating layer of a nickel oxide as a chief element as a base coating layer for a mixed catalyst of rare earth elements, such as lanthanum and cerium, and a precious metal (Patent Literature 3)    (4) An activated cathode comprising silver and a platinum group metal (Patent Literature 4)    (5) An activated cathode comprising 3 elements of platinum, ruthenium, and cerium (Patent Literature 5)    (6) An activated cathode comprising 3 elements of platinum, cerium, and lanthanum (Patent Literature 6)
Conventionally, the following activated cathodes for hydrogen evolution have been publicly opened as those by the thermal decomposition process.    (7) A mixed catalyst of ruthenium and cerium being manufactured in presence of oxalic acid. (Patent Literature 7)    (8) An activated cathode applying ruthenium nitrate and lanthanum carboxylate (Patent Literature 8)    (9) An activated cathode with an alloy of a transition metal like nickel and platinum or amorphous materials deposited on a conductive substrate (Literature 9, 10, and 11)
The activated cathode for hydrogen evolution manufactured by the thermal decomposition process applying precious metals as catalyst, as above, may be satisfactory in performance, but there is the problem in cost and it is essential to decrease the amount of precious metals used. In this case, however, thickness of the catalyst layer is small, and the overvoltage performance degrades within a normal life period of a cation exchange membrane or the substrate tends to dissolve. In addition, decreased amount of catalyst tends to accelerate deterioration of electrolysis performance at a high current density due to consumed catalyst.
Moreover, in this kind of an activated cathode for hydrogen evolution, an initial value of hydrogen overvoltage is high, and in order to keep it low for a long-time stable operation, further improvement is still required. In particular, in case of ruthenium applied as precious metal, a disadvantage lies in the fact that catalyst element consumes during a cease of operation for short-circuiting. Furthermore, in a chlor-alkali electrolysis cell by an ion exchange membrane process and the like, overvoltage performance tends to degrade easily when the electrolytic cell is contaminated with impurities in electrolyte.